The present invention relates to a noble dispersant for iron contained in a boiler water.
In a boiler plant, the iron contents in a boiler feed water are oxidized by dissolved oxygen, and are converted into iron oxide. It is well known that when iron oxide is deposited on the inside wall surface of the boiler plant, corrosion, heat-transfer hindrance, embrittlement accident and other hindrances are caused thereby.
It is customary that raw water of softened water is fed to a boiler operated under a low pressure of 20 Kg/cm.sup.2 or less. As the raw water or softened water generally contains iron, said iron contents are oxidized in the low pressure boiler plant, and deposited on the inside wall surface of the plant in the form of Fe.sub.2 O.sub.3. In order to prevent this problem, it has usually been devised to add polyacrylate as a dispersant to the boiler feed water to discharge the iron oxide, which would be deposited within the boiler if said dispersant was not used, out of the system in such a state as dispersed in the water.
In a boiler plant operated under a pressure of 20 Kg/cm.sup.2 or more, the thermal load exerted on the boiler is high and the impurities in the feed water extert many influences. Therefore, a demineralized water is used as the feed water in this instance, That is, there has been considered the countermeasure wherein the iron contents in the feed water are removed as much as possible before the feed water is fed to the boiler plant, and further the condensed water is fully treated thereby preventing the iron contents from being introduced into the boiler.
In spite of the above mentioned countermeasure, however, it is impossible to prevent the introduction of iron contents into the boiler completely, and to elude the occurrence of deposits in the boiler. It is known that 80-90% of said deposits are iron oxide in the form of Fe.sub.3 O.sub.4. Therefore, it is normal in the maintenance of a boiler plant to use demineralized water as the boiler feed water and to subject the boiler plant to acid cleaning once every few years.
As described above, in the boiler to which raw water or softened water is fed, polyacrylate as a chemical for dispersing the iron contents in the boiler water. The dispersion effect of polyacrylate is usually used is surely in the practical range but is not satisfactorily enough. And, referring to the boiler to which demineralized water is fed, the dispersion effect is low, namely about 1-30%, even when using polyacrylate. This may because the deposits take the form of Fe.sub.2 O.sub.4. In addition to this, addition of polyacrylate is disadvantageous in that the pH of the boiler water is raised over the standard criteria (ex Japanese Industrial Standards B8223).
In Japanese Patent Publication No. 6154/1986, there is disclosed a water soluble high molecular compound having a repeating unit as shown below and used as a boiler water treating agent. ##STR1## wherein, X stands for a hydrogen atom or a methyl group, A stands for acrylamide, methacrylamide, acrylic ester or methacrylic ester, and l and m stand for mol % of repeating units, l being in the range of 10-100, m being in the range of 0-90.
Japanese Patent Publication No. 6154/1986 teaches that said water soluble high molecular compound displays superior oxygen removal action and scale inhibiting action in the boiler water, but it is utterly silent about prevention or suppression of iron oxide deposit within the boiler. Likewise, Japanese patent application Laid Open No. 94482/1979 discloses a terpolymer having a repeating unit as shown below as a scale inhibitor, but is also utterly silent about prevention or suppression of iron oxide deposits within the boiler. ##STR2## wherein, X and Y each stands for a hydrogen atom or a methyl group, Z stands for a hydrogen atom, sodium, potassium, ammonium, hydrazium or an organic ammonium, A stands for acrylamide, acrylic ester or methacrylic ester, and l, m and n stand for mol % of repeating units in the relation of 5.ltoreq.l.ltoreq.90, 5.ltoreq.m.ltoreq.90 and l+m+n=100.